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4. Referans4: P. C. Eklof and J. Trlifaj, How to make Ext vanish, Bull. London Math. Soc. 33(1) (2001), no. 12, 41-51.
5. Referans5: J. R. Garcia Rozas and B. Torrecillas, Relative injective covers, Comm. Algebra 22(8) (1994), 2925-2940.